7 th ASIA AFRICA. Proceedings MINERAL RESOURCES CONFERENCE November University of Yangon, Yangon, Myanmar

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1 7 th ASIA AFRICA MINERAL RESOURCES CONFERENCE November 2017 University of Yangon, Yangon, Myanmar University of Mandalay, Mandalay, Myanmar Proceedings

2 QUARTZ VEIN TEXTURES IMPLICATION FOR ORE CLASSIFICATION IN KENCANA DEPOSIT, GOSOWONG GOLDFIELD, INDONESIA K. S. Electricia¹, Mega F. Rosana¹, Euis T. Yuningsih¹, S.N. Viqnoriva², Ildrem Syafrie¹, Hiroharu Matsueda 3 ¹Faculty of Geology, Padjadjaran University Jl. Raya Bandung Sumedang Km. 21, Jatinangor, Sumedang 45363, Indonesia ²PT.Nusa Halmahera Minerals Halmahera Island, North Maluku, Indonesia 3 Hokkaido University Museum Hokkaido University, Sapporo, Japan steamy4380@gmail.com INTRODUCTION The Gosowong Au-Ag district located in Halmahera Island, North Maluku Province, Indonesia. Gosowong goldfield are owned and operated by PT. Nusa Halmahera Minerals, a joint venture between PT. Aneka Tambang Tbk and Newcrest Mining Limited. The location of Gosowong goldfield is presented in Figure 1. Gosowong district has three deposits namely Gosowong (open pit), Toguraci (open pit then underground) and Kencana (underground) as shown in Figure 2. Kencana underground gold mine is hosted by two main sub-parallel North-West trending fault structures, named Kencana One (K1) and Kencana Two (K2) with strike lenght up to 600m, with vertical extent of 250m, and dipping Eastward, true width ranges from 1m to 20m (Coupland, 2009). Both structures joined by link structures, appears to be Kencana Link (K-Link) as the thickest link structure (Figure 3). Underground development of Kencana mine commenced in February 2005 with the first underground ore mined in March K1 has a multiphase formation history with numerous brecciation and opening events (Clark, 2012). As financial year ending in June 2016, Gosowong yield 197,000 Oz and since operating in 1999 over 4 moz of gold have been produced, K1 contribute over 20% of overall deposit (Newcrest, 2016). Kencana is a complex deposit with extreme high-grade gold occurences. Gold content can change drastically within a borehole as well as in underground face sampling results. Gold content often different from model interpretation, due to ground condition. The aim of this study is to determine correlation between quartz vein textures and quartz vein infill structures mode towards gold grade content, and to identify the distributions of textures and infill structures mode. Such identification might be apply as reference to the original conditions in the ground. This approach can guide geologists in the field to determine mine direction with quartz vein textures and gold content. This study focus on K1, due to it has the most complete information as mining process is in progress and underway for a long time. Figure 1. Gosowong Operation location (Modified after Coupland, 2009) 1

3 Figure 2. Orientation of geological structure in Gosowong (PT.NHM, 2010) K1 A B Figure 3. Kencana deposit, (A) West view, (B) North view; K1-light green, K2-red, K-Link- yellow (PT.NHM, 2015). 2

4 GEOLOGY AND MINERALISATION Halmahera Island is a part of an active island arc that is situated to the east of the Molucca Sea Plate, where the opposing Sangihe and Halmahera arcs have been actively converging since the late Pliocene (Richards et al., 2004). Plio-Pleistocene volcano-sedimentary succession occurs immediately adjacent to Quaternary or Recent volcanic cover rocks were considered prospective for epithermal deposits (Carlile et al., 1998). At Kencana, epithermal mineralisation occurs at or immediately adjacent to the contact between andesite-dominated and underlying basalt-dominated units of the Gosowong Formation (Fitzpatrick et al., 2015) as shown in Figure 4. The K1 structure strikes in Northwest-Southeast direction, 315 from North, dipping in towards Northeast. Kencana s stratigraphy demonstrate the Quaternary late stage, as mineralization took place in andesitic package, predominantly andesite lava, volcaniclastic mudstone on the top of volcaniclastic conglomerates, and diorite as intrusion. Figure 4. Stratigraphy of Gosowong and surroundings (PT.NHM, 2015). Kencana has an extreme high-grade gold occurences at depth, as some are presented in Table 1. The main mineralization zone characterized by massive to brecciated chalky quartz adularia sulphide banded veining overprinted by dark green chlorite-sulphide banded veining locally and late stage black sulphide of colloform banding-cockade textures, grading over 80 g/t Au. The sheared zone mineralization characterized by moderate to intense shearing, breccia and clay after adularia, contains wallrock and main zone fragments. It forms mostly at the top of the main zone as hanging wall zone and other location within the package, typically grading g/t Au. Stockwork and sheeted mineralization occurs in significant volumes with gold grades typically moderate to low gold grades, g/t Au. Table 1. Gold grade content within one drill hole, i.e. KSU033 (PT.NHM, 2015) Hole ID From (m) To (m) g/t Au KSU

5 METHOD The resource definition process applied at Gosowong is based on systematic diamond drilling with some Reverse Cirulation (RC) drilling used when appropriate. It has been established that shoots drilled spacing of approximately 50x50 metres can generally be classified as Inferred Resource. Drill spacing 25x25 metres can support Indicated Resource classification. Due to the extremely high grades presence at Kencana deposit, it is need to drill with spacing 12.5x12.5 metres or equivalent for grade control purpose. Diamond core is logged and photographed prior to sampling for fire assays. Sampling protocol based on heterogenity testing undertaken from time to time, generally based on half core. Core recovery is monitored carefully. Samples are assayed for gold and silver grades at on site laboratory using screen fire assay method. PT. Intertek Utama Services (ITS) as onsite laboratory conduct screen fire assays for Au-Ag. Final results input directly to database to generate intercept. Intercept represent gold contents at depth within one drill hole (Figure 5). Then, quartz vein description applied on screen running by certain software to determine its characteristic, quartz vein texture and infill structure as well as quartz percentage. Gold contents within quartz vein identification would drives for next drill plan prepared by geologist. A total of 140 drill holes were used in this study consists of exploration, resource definition and grade control holes, 95 drill holes conducted from underground and 45 drill holes drilled from surface. Total of 2938 samples data extracted from 140 drill holes are reviewed for this study. All data were sorted by its gold content, and then sorted by its quartz vein textures with consider the quartz percentage, classified into several ore body classification subsequently. This method will demonstrate dispersion of quartz vein textures and infill structures. Data processing steps consist of: a. Gold content is sorted into several classification based on condition of the ground in terms of production and used for gold grade blending before entering the mill. b. Each classification sorted by its quartz veins textures and quartz vein infill structures mode subsequently. RESULTS AND DISCUSSION Ten major types of quartz vein textures were recognized in Kencana and its surrounding (Figure 6) with five types of quartz vein infill structures occurs. Quartz vein texture is used for identification of general physical appearance or character of a rock, including the size and shape, and the mutual relations among its component minerals. Dong et al. (1995) cited quartz vein infill structure is generally used for the larger features of a rock and is determined by the spatial arrangement of its mineral aggregates which differ from one another in shape, size, composition, and texture. Ore body were classified into five group, each group exhibit gold grade range as well as infill structures and consists several quartz vein textures, respectively. The classification are: a) Black colour represent gold grade g/t Au, consider as low grade b) Yellow colour represent gold grade g/t Au, consider as medium grade c) Red colour represent gold grade g/t Au, consider as medium grade d) Pink colour represent gold grade g/t Au, consider as high grade e) Blue colour represent gold grade >80g/t Au, consider as high grade. Quartz vein textures recognized in Kencana are (Figure 6): 1. Bladded (Bd), lattice calcite pseudomorph intersecting replaced by quartz, aggregates of quartz or chalcedony may be arranged in a bladed or platy form. 2. Crystalline (X), pointy shape crystals, display alternating clear or vitreous and milky zones within individual quartz crystals. 3. Drussy/Comb (Cm), refers to group of parallel or subparallel quartz crystals which are oriented 4

6 perpendicular to vein walls, resembling teeth of a comb. Crystals displays a uniform grain size and have euhedral point at their free ends. 4. Saccharoidal (Sa), vitreous to milky fine-grained quartz aggregates have the appearance of sugar. 5. Massive (Ma), a general term refers to quartz veins which have more or less homogeneous appearance over wide areas and display absence of banding or layers, shear or fractures. 6. Moss (Mo), silica aggregates display a heterogeneous turbid appearance, groups of spheres ranging from mm in diameter. It may shows gradation to colloform texture if the spheres become interconnected. 7. Crustiform (Cr), chalcedony and quartz finely layered subparallel, narrow, commonly symmetrically developed from both walls of a fissure, with slightly jagged formations and distinguished by differences in textures, mineral proportions, and color (Dong et al., 1995). 8. Cryptocrystalline (Ct), chalcedony and quartz finely layered and surrounds a fragment locally. 9. Colloform (Co), chalcedonic and quartz together with sulphide aggregates shows both spherical and mammillary forms. 10. Cockade (Ck), colloform texture and layers surrounds a fragment, can be quartz vein flake and or wall rock. 3. Red colour g/t Au, predominantly by cryptocrystalline with minor sulphide banding, crustiform texture and moss texture. 4. Pink colour g/t Au, predominantly by colloform - sulphide banding, cryptocrystalline with some sulphide banding textures and cockade textures subsequently. 5. Blue colour - >80g/t Au, predominantly by cockade-sulphide-chlorite banding and colloform banding subsequently. This study exhibit that ore body with high grade gold content dominated by cockade and colloform textures. Cryptocrystalline texture increasing toward the medium-high grade ore body. Moss and crustiform textures occurs in medium grade ore body are quite persistence, follows by massive and saccharoidal that occurs in medium and fading toward main ore body (Figure 7). Gold content classification associated with quartz vein textures distribution is presented in Table 2 and Figure 7 are: 1. Black colour g/t Au, predominantly by quartz crystalline textures, followed by comb/drussy textures as well as cryptocrystalline and crustiform textures. 2. Yellow colour g/t Au, predominantly by quartz crystalline and cryptocrystalline textures, and moss texture thereafter. 5

7 10100E 10000E 9900E Au 4800m Au Au Au Au Section 19900N Figure 5. Intercepts generated by drilling result, i.e in section North. (PT.NHM, 2015) Figure 6. Quartz vein textures in Kencana and its surroundings (PT.NHM, 2009). 6

8 Frequencies Table 2. Correlation between gold content and presence of quartz vein textures frequencies. Classification colours g/t Au Quartz vein texture Bd X Cm Sa Ma Mo Cr Ct Co Ck > Quartz Vein Textures >80 Gold grades Bd X Cm Sa Ma Mo Cr Ct Co Ck Figure 7. Quartz vein textures frequencies toward gold grade content in K1, Kencana. Abbreviations: Bd: bladded; X: crystalline; Cm: drussy/comb; Sa: saccharoidal; Ma: massive; Mo: moss; Cr: crustiform; Ct: cryptocrystalline; Co: colloform; Ck: cockade. Open space filling is the characteristic of epithermal vein systems, especially in Kencana they are controlled by geological structure, and appear to fill up the available open space. Evidence of fluid boiling is present in Kencana, as well as hydraulic fracturing that may create breccia filled with vein material. Crack-seal veins appear subsequently in order of millimeters, identified as stockwork and cavity filled locally. Data distribution demonstrate positive skewness in relative terms, as ore classification g/t Au has more data than the g/t Au. This is the characteristic of K1-Kencana that medium to high gold content are quite intense with its extreme changes of gold content (Table 3 and Figure 8). Distributions of precious metals are always positively skewed, as most of the samples report is low grades with a small percentage of extreme high grades (Coombes, 2008). The relevance can be seen in the distribution of 2938 samples data retrieved from drill hole used for this study is presented on Table 3 and Figure 8. Table 3. Samples data of KI used for this study Gold content (g/t Au) Samples number > Total

9 K1 Samples number distribution >80 Gold grades (g/t Au) Figure 8. Sample distribution of K1 - Kencana Moncada et al in 2009 done a study to determine if there is a correlation between gold and silver grades and the presence and abundance of fluid inclusion and mineralogic features characteristics of boiling in order to develop a simple technique that can help explorationists to distinguish between barren and mineralized veins. This study indicate the texture that has best relationship to high Au grades is colloform-cockade silica texture as feature of boiling system. The relevance can be seen in the quartz vein textures frequencies distribution in Figure 7. PT. NHM has made a picture of the spread of quartz texture from top to bottom in 2010 to identify quartz vein texture distribution, is presented on Figure 9. This study is supplemented by data representing the area K1 and indicates that the trend of high-grade gold quartz texture dominated by cockade and colloform textures. CONCLUSIONS AND RECOMMENDATION Cockade-colloform texture dominating in high grade gold content ore body, accompanied by wallrock breccia as quartz vein infill structure. Medium gold content within ore body dominated by quartz vein texture cryptocrystalline and crustiform subsequently, wallrock breccia dominate in quartz vein infill structure. Low grade gold content within ore body has strong crystalline texture followed by cryptocrystalline, and stockwork-sheeted dominating as quartz vein infill structure mode. This study is expected to be a guide in the implementing of mining in underground in order to optimize mining practice, armed with field data has been learned exhibit these features can be used as reference. Mine geologists could possibly more confident determine ore drive direction based on quartz vein textures and quartz vein infill structures mode prior the assays result return. Face of underground condition illustration can be seen on Figure 10. Quartz vein textures distribution downdip, 2010 Quartz vein textures distribution downdip, 2017 Bladded-lattice calcite pseudomorph ± crystalline; stockwork. Crystalline ± drussy/comb ± bladded-lattice : stockwork, breccia Crustiform ± Bladded-lattice calcite pseudomorph ± crystalline; sheeted, stockwork. Crystalline ± drussy/comb ± bladded-lattice ± moss ± massive ± saccharoidal ±crustiform ± cryptocrystalline; stockwork, solid, wallrock breccia and vein material breccia. Figure 9. Distribution of quartz vein texture in K1 downdip, modified after PT.NHM,

10 Figure 10. Face of underground mining of Kencana deposit. ACKNOWLEDGEMENTS Thanks conveyed to PT.Nusa Halmahera Minerals - entire team Mineral Resource Department for the opportunity to conduct this research. The authors gratefully acknowledge Dadan Wardiman and Colin McMillan for guidance during the completion of the research. REFERENCES Carlile, J.C., Davey, G.R., Kadir, I., Langmead, R.P., and Rafferty, W.J., Discovery and exploration of the Gosowong epithermal gold deposit, Halmahera, Indonesia. Journal of Geochemical Exploration, 60, h Clark, L.V., The geology and genesis of the Kencana epithermal Au-Ag deposit, Gosowong Goldfiels, Halmahera Island, Indonesia. Ph.D Thesis at CODES University of Tasmania, h Coombes, J., The Art and Science of Resource Estimation. Coombes Capability, Perth, 218 h. Coupland, T., Sims, D., Singh, V., Benton, R., Wardiman, D., and Carr, T., Understanding geological variability and quantifying resource risk at the Kencana Underground Gold Mine, Indonesia. Seventh International Mining Geology Conference, h Dong, G., Morrison, G.W., and Jaireth, S., Quartz Textures in epithermal veins, Queensland classification, origin, and implication. Economic Geology, v.90, h Fitzpatrick, N., Harris, A., MacCorquodale, F., and Wardiman, D The Gosowong Goldfield - A World class Epithermal Gold-silver District Indonesia. Pacrim 2015 Congress, Hong Kong, China, h Moncada, D., Bodnar, R.J., Reynolds, T.J., Rimstidt, J.D., and Mutchler, S., Characteristic and distribution of mineral textures and fluid inclusions in the epithermal Ag-Au deposits at Guanajuato, Mexico. Geoscience at Virginia Tech, abstract: Richards, T.H., Basuki, D. and Priyono, M.D., Discovery of the Toguraci epithermal Au-Ag deposits, Gosowong Goldfield, Halmahera Island, East Indonesia. Proceedings PACRIM 2004 The Australasian Institute of Mining and Metallurgy: Melbourne, h